In recent years, along with the increase in the operating speed of personal computers, the spread of network infrastructure, the markedly increased capacity of data storage, and the decrease in price, a further opportunity is increasing in which information such as text and images which have traditionally been supplied in the form of paper printed matter is now acquired and read as convenient electronic information.
Employed as such electronic information reading means are conventional liquid crystal displays as well as CRTs, and in recent years, emissive displays such as organic EL displays are drawn attention. Particularly, when electronic information is in the form of text, it is necessary for an operator to watch the browsing means for a relatively long period of time, which is not a human-conscious operation. Generally, it is known that emissive displays exhibit the following disadvantages: screen flickering results in eye fatigue; display devices are not portable; reading posture is limited; operators tend to gaze at a still screen; and reading over an extended period of time results in an increase of electric power consumption.
Known as display means to overcome such disadvantages are reflective displays (having a memory effect) which do not consume power for maintaining images since these displays use external light. However, due to the following reasons, it is difficult to mention that sufficient performance has been achieved.
Namely, displays employing polarized plates such as reflective liquid crystals exhibit reflectance as low as approximately 40 percent, resulting in having a problem in displaying white. Further, the production methods employed to prepare constituting members are not simple. Polymer dispersion type liquid crystal displays require relatively high voltage, in addition, since only the difference in the refractive index among organic materials is utilized, the resulting images do not exhibit sufficient contrast. Polymer network type liquid crystal displays result in problems in which a high voltage is required and in order to enhance memory functions, complicated TFE circuits are required. Display elements based on an electrophoretic method necessitate voltage as high as at least 10 V, and durability problems may occur due to coagulation of electrophoretic particles. In order to use one of the above methods for a color display, a method to use a color filter or a method to used a pattern color have been known. In the former method, it is difficult to obtain a bright white display, in principal, due to the coloration of the color filter, and the latter method has a problem in that a dark black color cannot be obtained due to the use of a patter color.
As one of the methods which enable a full color display, an electrochromic method has been know, which can be driven with a voltage of 3V or less. When a display exhibiting a bright white color, a high white-black contrast and a color display is prepared using an electrochromic method, it is necessary to laminate three layers each exhibiting a different color, resulting in a high cost due to the complicated element constitution. A full color electrochromic display having a flat color mixing structure has been known (for example, refer to Patent document 1). In this method, no fully sufficient white-black contrast has been obtained, since a dark black color cannot been obtained due to the flat color mixing structure. Also, a method to use a poly pyridine compound in an electrochromic display has been known (for example, refer to Patent document 2), however, in this method, only two colors can be displayed and, specifically, a black display cannot be obtained.
As a method which enables black display, an electro deposition method (hereafter, referred to merely as an ED method) has been known, in which dissolution-deposition of a metal or a metal salt is used for the display (for example, refer to Patent document 3). An ED method is an excellent method to obtain a sufficient white-black contrast, however, color filters of B, G and R, or Y, M and C are needed in order to display a full color, whereby the excellent white-black contrast has not been fully utilized.
Patent documents 1 Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2003-270670
Patent documents 2 Japanese patent No. 2930860
Patent documents 3 JP-A No. 2003-241227